Production of cast iron



nit States atent 3,007,791 Patented Nov. 7., 1961 fiice This invention relates to methods of producing cast 1ron.

Objects of the invention include the provision of an improved oast iron process by which it shall be possible to produce parts of a large variety of dimensions and shapes having a high degree of machinability and having reproducible physical and mechanical characteristics of a high order. Another object is to provide a method of producing cast irons of such characteristics as to render them suitable for use as desirable substitutes for various grades of semi-hard steel and alloy iron in cases where the latter materials have heretofore been considered necessary.

The manufacture of iron castings of good mechanical characteristics has heretofore been beset with considerable difliculties especially in the case of castings of relatively intricate shape with sharp variations in thickness dimensions. Cast iron parts as produced by the processes available to the art have been difficult or impossible to machine, have been liable to show porosity rendering them permeable to gas and vapours, have displayed poor wear resistance due to lack of structural homogeneity, the castability of such irons has frequently been poor and they were prone to cracks, blowholes and other defects. A great number of attempts have been made in the past to improve the characteristics of cast iron. Some of these prior attempts have involved modifications in the composition of the iron by the addition to the melt of various ingredients including such costly metals as chromium, nickel, molybdenum and the like. Other attempts have been concerned with the methods of charging and operating the melting furnaces, and closely controlling the uniformity of successive melts. While many of such measures may be partially successful, there has been no sure way so far available consistently and positively to eliminate the above enumerated difiiculties and defects.

A close analysis of the conditions prevalent in cast iron processes now in use and extensive experimentation have now shown the applicant that all of the iron melting procedures currently used are open to a common defect, in that the rate of reduction of the oxides of the melt is too slow owing to the presence in the melting furnace of an excessively oxidizing atmosphere. These conditions undesirably modify the melt constituents, resulting in combustion losses for some of the constituents, causing non uniform concentration of other constituents such as phosphorous, which may be brought to objectionably high concentrations, and producing inclusions by dissolving various gaseous and other impurities in the molten metal.

On the other hand, it is well known that a CO-rich reducing atmosphere is difficult to maintain in a melting furnace, especially when high temperatures are required, in which cases the diagram of operation of the furnace (temperature versus time) is representable by a sawtooth curve due in the main to the highly exothermal character of the reaction CO +C22CO above 1100 C. Corresponding large amplitude variations are reflected in the chemical composition and physical characteristics of the metal. All the means heretofore suggested for regulating the operation of the furnaces and minimizing such variations have been empirical, and have not made it possible to provide the desired operating uniformity to the extent required in industrial processes and products.

My investigations have shown that the desired uniformity in iron melting processes can consistently be achieved when the melting operation is performed in a highly reducing atmosphere containing a suitable proportion of one or more ketenes.

While various ketenes are usable according to the invention as reducing agents for iron melting operations, especially good results have been obtained with the lower carbon oxide having the formula which shows extreme avidity to oxygen, and is highly unstable at elevated temperatures, in contrast with carbon monoxide CO which becomes stable at temperatures above 1100 C. If therefore the melting furnace atmosphere is so regulated as to maintain therein a pseudoequilibrium between the ketene and carbon monoxide, there will at all times be made available an adequate quantity of carbon monoxide, in other words there Will be obtained the requisite reducing atmosphere which, among other beneficial effects, will greatly reduce or completely eliminate the temperature irregularities that have been the chief sources of the difiiculties encountered in conventional cast iron processes and the defects in the cast iron products.

In conducting my tests I have found that when using ketene-containing atmospheres the sulfur present in the melt and in the fuel is converted to carbon disulfide CS a part of which passes into solution in the metal. A similar reaction occurs with the silicon present in the melting charge, yielding silicon disulfide SiS The carbon and silicon disulfides thus generated have an influence upon the carbon in graphite form. In the presence of such sulfides the graphite is more abundant and concentrated, and the total sulfur content may be substantially increased Without any danger of causing distemper or white iron. Such action of the sulfides does not, however, modifiy the form of the graphitization curve. The graphitization curve obtained when performing the melting process in ketene-containing atmosphere closely follows, ceteris paribus, the total carbon curve. However, tests have shown that the graphitization curve may, if desired, be made to diverge widely from the total carbon curve, by applying suitable measures after the melting processes has been started, e.g. modifying the depth of the melting zone, modifying the proportion of fuel in the charges, and/or increasing the amount of ketenes in the furnace atmosphere.

The tests also point to the possibility of controlling the rate of primary graphitization. Moreover, on investigation of the curves derived from the tests, it has been discovered that the fusion curve in terms of carbon has the general shape of a shallow V, and the curve in terms of silicon is initially U-shaped and then continues as a regularly and slightly drooping line. From this it is inferred that, in order to obtain maximum uniformity in characteristics, the best procedure is not to maintain a constant charge, as was the universally accepted and recommended practice heretofore, but, instead, to vary the charges especially in regard to the concentrations of certain constituents, as a function both of time and tem perature during the melting process.

The above general principles apply, mutatis mutandis, both to high-carbon and low-carbon melts. Primary graphitization, now a fully controllable factor, will always be higher (for a given total carbon percentage) when working in a ketene-laden atmosphere than when using any other known melting procedure. The tests have shown that the same remains true even with very low total carbon concentrations, and similar results have been obtained when using 100% steel charges.

In the light of the foregoing, the method of melting metals, especially-cast iron, according to this. invention can .be. said to comprise the basic characteristic of operatingalnderlhighly reducing atmosphere, having a substantial ketene content, so as to establish a pseudo-equilibrium between the ketenes and carbon monoxide constituents.

The ketene-containing reducing atmosphere of the invention may be provided, for a normal range of proportions of coke and air and depth ofthe fusion bed, by the combustion of a. suitable gaseous or liquid fuel adapted to release the desired ketenes inthe melting furnace, e.g. in the area surrounding the blast nozzles or between the blast nozzles and the meltingzone. One suitable fuel composition which will release ketenes for the purposes of the invention comprises a mixture of acetone and benzol, in widely varying proportions as from /90 to 90/ 10 depending on the rate of feed. If desired, one or more of the factors including rate of blast air, depth of the melting -bed,.and proportion of coke in the charge, may bemade adjustable for optimum results as determined by preliminary testing.

The process of the invention may be performed in known melting furnaces of the cup-ola type, preferably fitted with auxiliary burner means for burning the ketenegenerating fuel. Such additional burners may be mounted adjacent to the air blast nozzles or between the nozzles and-the melting zone, or directly on the nozzles. The auxiliary burner means may assume the form of injection nozzles.

Preferably the blast air used in the furnace is enriched with oxygen, thereby increasing the output rate and further improving the uniformity of distribution of the carbon in the melt.

The ketene-releasing fuel used according to the invention may advantageously have admixed with it one or more additional elements in suitable proportion for improving the characteristics of the metal.

The process of the invention may be carried out as a batch process, with successive charges of identical or different composition for producing similaror different metal products. Alternatively the inventionmay be embodied as a continuous process. In such case also the successive charges introduced into the furnace may be identical for producing a constant product, or may be altered as from one charge to the next without stopping the melting operations in order to produce successive IneltS of different controlled characteristics.

According to a feature of the invention, in this latter case the successive charges may be predetermined in accordance with the varying characteristics required of the output metal by the following procedure. Preliminary testsare conductedto produce batches of metal having each of the desired sets of characteristics, and from the data derived from each testthe precise composition and proportions of each charge to be used is determined. If the desired amountof a given grade of metal is less than the capacity of the furnace used, the preliminary test data will make it possible to determine the time at which to change over from a charge of one composition to a charge having a different, likewise predetermined, composition, in order-.10 modify the grade of the product metal without any stoppage of the melting process.

In starting a melting process in accordance with the invention, it'ispossible .to start with a batch of ordinary cast iron, exhaust the batch completely, and then begin to control the conditions as. outlined above in accordance with the-particular grades of material to be produced. Alternatively, starting with a batch of ordinary cast iron, all the conventional adjustments corresponding thereto may. be effected, andthen the carbon content may be doubled by step by step additions of carbon content in the successive charges, the rate of such incremental additions being such that at each step the carbon content will have been able to assume a steady value before proceeding to the next step; throughout such additions care should be taken not to disturb the blast air adjustment; finally the supply of ketenereleasing fuelis started.

The sulfur content inthe metalchargesis desirably in the range of from about 0.02% to about 0.2%; the silicon content may be in the approximate range of from 0.10% to 2.5%.

In the previously described continuous process in which the composition of successive melts is varied, one convenient method of-ascertaining the precise times atwhich the melts are changed, is to add small mounts of suitable additions to the melt capable of highly exothermal reac tions therewith to produce bright sparks, which serve as luminous tracers providing visual indication of the change from one melt to the next.

The process of the invention is applicable to all melt compositions as presently known in the production ofcast iron. As already indicated .the. resulting cast iron products have greatly improved mechanical characteristics, equalling those of semi-hard steels. Thus, one typical set of mechanical characteristics in cast iron produced by the improved method is the following:

kg. load on standard cast testpieces 22 mm. thick. Brinell hardness 225435.

The cast iron articles have an extremely sound and compact structure free of porosity, blowholes and the like, are readily machinable, and their mouldability is excellent; castings having sudden changes in section from 4 mm. to 240 mm. have been produced with excellent results.

An important additional advantage of the inventionis that the above unexpectedly desirable results are obtainable from melts comprising a major proportion, up to and more, of rejects and scrap iron. The cast iron products-produced by the new method may include lamel lar graphite cast iron, as well as iron stock for steelworks and for any other purposes.

What I claim is:

1. In a method of melting iron, the step ofcreating a reducing atmosphere in contact with the melt by introducing at least one ketene into said atmosphere.

2. In a method of melting iron, generating a ketene in the atmosphere in contact with the melt.

3.111 a method of melting iron, introducing a ketene in the atmosphere contacting the melt in a proportion selected to maintain an equilibrium between carbon monoxide and ketene therein.

4. In a method of producing cast iron, introducing the V ketene 7. In a cast iron process, melting successive charges of cast non of predeterminedcompositions in a ketene containing reducing atmosphere.

8. In a continuous cast iron process, the steps of continuously introducing successive ferrous charges of predetermined compositions and continuously melting said charges in a ketene-rich reducing atmosphere.

9. In a cast iron process, the steps of continuously introducing successive ferrous charges of predetermined compositions and continuously melting said charges in a ketene-rich reducing atmosphere, adding a small amount of a spark-generating reagent to each charge to provide a visual indication of the commencement of each new melt, and adjusting the melting conditions in accordance with the composition of each charge at each such visual indication.

10. In a cast iron process, the steps of melting a ferrous charge, introducing further charges of increased carbon content in successive steps so timed that the carbon content in the melt resulting from the preceding step has reached a stable value, and then supplying a ketene-rich reducing atmosphere to and in contact With the successive melts.

11. In a method of producing cast iron, the steps of injecting oxygen-enriched blast air into, and providing a ketene in the atmosphere contacting the melt in a proportion Within a range such as to maintain an equilibrium 2O between carbon monoxide and ketene in said atmosphere.

12. In a method of producing cast iron, melting a ferrous charge containing sulfur in the approximate range 0.020.20% in a ketene-rich atmosphere.

13. In a method of producing cast iron, melting a ferrous charge containing silicon in an approximate range (HO-2.50% in a ketene-rich atmosphere.

14. In a method of producing cast iron, melting a ferrous charge containing about 10% iron and about scrap iron as the ferrous ingredients therein, in a ketenecontaining atmosphere.

15. In a method of producing cast iron, melting a ferrous charge containing about steel as the ferrous ingredient therein, in a ketene-containing atmosphere.

References Cited in the file of this patent FOREIGN PATENTS 578,023 Great Britain June 12, 1946 

1. IN A METHOD OF MELTING IRON, THE STEP OF CREATING A REDUCING ATMOSPHERE IN CONTACT WITH THE MELT BY INTRODUCING AT LEAST ONE KETENE INTO SAID ATMOSPHERE. 